The present invention, relates to a structure of a resin sealed electronic device and, more particularly to a package structure and configuration to improve its reliability.
The prior art of a semiconductor package structure for an internal combustion engine, particularly of an igniter, as disclosed in Japanese Patent No.2,590,601, proposes to seal a circuit board and a semiconductor element with transfer mold. Further, Japanese Patent Application Laid-Open No.9-177647 discloses an igniter which is formed in one chip by constructing a function circuit on a semiconductor chip.
In a case where a hybrid IC board and a semiconductor power element are mounted on a single heat sink portion and sealed with transfer mold, it is particularly necessary to pay attention to thermal stresses acting on the contained parts. Since an amount of thermal deformation is large particularly in the portion of the semiconductor element having a large amount of self-heat generation, there occurs a problem in lifetime of solder used for mounting the semiconductor power element. Silicone which is a main material composing the semiconductor power element has a linear expansion coefficient of 3xc3x9710xe2x88x926/xc2x0C., and on the other hand the metallic heat sink (usually made of a copper group material) mounting the semiconductor power element has a large linear expansion coefficient as large as 17xc3x9710xe2x88x926/xc2x0C. Therefore, large stresses such as a shear stress caused by the difference between the linear expansion coefficients and a bending stress caused by the bimetal effect occur in the solder used for mounting the power element. In addition, in a case of containing a hybrid IC board, thermal stresses acting on the board must be also taken into consideration. In a case of using a hybrid IC board made of an alumina group material, the linear expansion coefficient is around 7xc3x9710xe2x88x926/xc2x0C., and is also different from the thermal expansion coefficient of the heat sink. Therefore, it is necessary to improve the lifetime of the resin sealed electronic devices by optimizing specifications of the linear expansion coefficient of the resin used; the method of coating the surface of the semiconductor power element; and the heat sink mounting the board and the semiconductor power element so as to reduce the effect of thermal stresses acting on the contained parts.
In order solve the above problem, by using a switching semiconductor element of which the protective film on the power element is coated with a polyimide group or a polyamide group resin when the used semiconductor power element is manufactured, tightness of adhesion of the semiconductor power element to the transfer mold resin is improved. By doing so, the thermal displacement of the semiconductor power element is restrained to reduce the thermal stress acting on the soldered portion. In general, a semiconductor element is provided with a protective film of some kind at the end in manufacturing. Therefore, it is easy that a protective film made of a polyimide group or polyamide group resin is employed as the protective film in that manufacturing process. On the other hand, there is a method to improve the restraining force by dropping and curing a resin of such kind after mounting the semiconductor element. However, the method increases number of manufacturing processes to increase its cost.
In general, in order to mount a semiconductor element, a metallic heat sink has nickel plating on its mounting surface or additionally has silver plating or the like on the nickel plating. There are two plating methods, that is, one is that the heat sink is plated after being pressed into a part-mounting shape, and the other is that the heat sink is plated before being pressed. These kinds of plating are poor in adhesion to the resin, and consequently weak in a force restraining the heat sink portion. Particularly, in the case of plating after being pressed, it is difficult to plate only the mounting area, and accordingly all over the surface is plated. From the viewpoint of thermal deformations of the semiconductor parts, the board and the heat sink, in order to improve reliability a method of restraining these contained parts is employed by fully molding all the parts with transfer mold. However, when all the surface is plated, separation of interface easily occurs particularly in the reverse side surface of the area not mounting the parts due to the poor adhesion with the resin, and as a result the thermal deformation can not be restrained and accordingly the reliability can not be improved. In order to solve the problem, it is considered that the polyimide group or the polyamide group resin is applied to the surface or that the surface is coated with the resin as the semiconductor power element. However, the method adds number of manufacturing processes such as applying and curing processes.
Therefore, the heat sink is plated when it is in its material state, and the reverse side surface of the surface mounting the parts is masked so as to be not plated. The base material surface of the metallic material (a copper group or an aluminum group material) is tighter in adhesion to the resin compared to that of the plating described above, and consequently the thermal displacement can be restrained.
As described above, the semiconductor parts and the board are actually mounted on the surface of mounting the parts, and the exposed plated area having a weak adhering force is small. Therefore, tightness of adhesion of the semiconductor power element to the resin is maintained by coating the surface of the element, and tightness of adhesion of the reverse side surface to the transfer mold resin is improved by the base material surface of the metallic material without the plating. By doing so, the thermal displacement of the portion of the semiconductor power element is reduced by restraining the upper and the lower surfaces. On the other hand, in regard to the portion of the hybrid IC, the difference of the linear expansion coefficients between the substrate material of the aluminum group ceramic and the metallic heat sink material (the copper group or the aluminum group material) is small compared to the difference in the portion of the power element. Therefore, the reliability can be improved by improving the tightness of adhesion of the reverse side surface of the heat sink to the resin to restrain the displacement in the heat sink side.
In order to improve the tightness of adhesion of the reverse side surface to the resin, it is possible to further improve tightness of adhesion in the interface by forming projections and depressions on the reverse side surface of the heat sink portion to increase the contact area.
In that case, the thermal stresses acting on the contained parts can be reduced by using a mold resin having a linear expansion coefficient within a range of the smallest linear expansion coefficient (3xc3x9710xe2x88x926/xc2x0C.) of silicon in the semiconductor element among the constituent materials of the contained parts to the linear expansion coefficient (17xc3x9710xe2x88x926/xc2x0C.) of copper among the metallic materials (the copper group or the aluminum group material) to be used for the heat sink which has the largest volume.